The present invention relates to an optical-fiber wiring board and an optical-fiber wiring assembly for optically interconnecting optical elements, optical circuits or optical apparatuses, and more particularly to an optimal optical fiber for the optical-fiber wiring board.
Conventionally, an optical-fiber wiring board is used to optically interconnect optical elements, optical circuits or optical apparatuses. The optical-fiber wiring board is constructed to include a plurality of optical fibers routed on a substrate in a predetermined pattern. Generally, a communication-dedicated single mode optical fiber having an outside diameter of 250 xcexcm is used as an optical fiber that is routed on the optical-fiber wiring board. For example, Japanese Unexamined Patent Publication No. 2000-66035 discloses an optical fiber having an outside diameter of 125 xcexcm for an optical-fiber wiring board.
In the optical-fiber wiring board, a number of portions in which the optical fibers are bent and routed. As such, the bending loss of the optical fibers is increased, and the performance of the optical-fiber wiring board is thereby lowered. Hence, for the optical-fiber wiring board, the optical fibers need to be routed not to increase the bending loss. However, a problem arises in that such a routing manner makes it difficult to implement the miniaturization of the optical-fiber wiring board.
In more specific, on the optical-fiber wiring board, linear portions where the optical fibers are routed to be linear on the substrate and curved portions where the optical fibers are bent and routed to be curved. In order to prevent the increase in the bending loss of the optical fiber in the curved portion, the curved portion needs to be formed to have a minimum radius of curvature, specifically, a radius of curvature greater than a minimum radius of curvature allowable with respect to the bending loss. However, in order to form the curved portion with the radius of curvature greater than the minimum radius of curvature, the area of the substrate needs to be proportionally increased. Consequently, the size of the optical-fiber wiring board is enlarged.
In addition, on the optical-fiber wiring board, crossover sections are formed in each of which two optical fibers are routed such that the one optical fiber (upper optical fiber) crosses over the other optical fiber (lower optical fiber) routed in contact with the surface of the substrate. In this crossover section, the upper optical fiber is routed to be bendable. In order to reduce the bending loss of the upper optical fiber, the upper optical fiber needs to be routed to have a radius of curvature greater than the minimum radius of curvature. However, when the upper optical fiber is routed to have a radius of curvature greater than the minimum radius of curvature, mutual interference occurs between the upper optical fiber and other optical fibers routed adjacent to the lower optical fiber routed on the substrate. In order to prevent the interference, the wiring density of the optical fibers on the substrate needs to be reduced. In order to achieve the reduction, however, the area of the substrate needs to be enlarged, which consequently leads to enlargement of the size of the optical-fiber wiring board.
When the number of the crossover sections formed on the optical-fiber wiring board is reduced, the bending loss on the overall optical-fiber wiring board is reduced. However, the reduction in the number of the crossover sections cannot be implemented unless otherwise the area of the substrate is enlarged. As such, the size of the optical-fiber wiring board needs to be enlarged to reduce the number of the crossover sections.
Thus, with ordinary communication-dedicated optical fibers used with a device such as an optical-fiber wiring board on which the fibers are bent and routed, the bending loss is increased. When the optical fibers are routed considering the bending loss to prevent the problem, another problem arises in that the size of the device using the optical fibers is enlarged.
The present invention is made in view of the above-described circumstances, and an object thereof is to provide an optical fiber suitable for use in an environment including many portions where the optical fiber is bent. More particularly, the present invention is to provide an optical fiber that is optimal for use with an optical-fiber wiring board, and an optical-fiber wiring board and an optical-fiber wiring assembly that use the optical fiber.
In order to achieve the above-described object, the present invention has been accomplished by paying attention to the fact that an optical fiber used with an optical-fiber wiring board or the like has an overall length of about 0.5 to 1 m, which is considerably small in comparison with the length of an optical fiber used for communication. Specifically, the present invention has been accomplished by paying attention to the fact that dissimilar to an ordinary communication-dedicated single mode optical fiber, such as a communication-dedicated single mode optical fiber according to the ITU (International Telecommunication Union) standards, since the length of the optical fiber used with the aforementioned optical-fiber wiring board or the like is short, the dispersion need not be taken into consideration as a design parameter.
In specific, a first aspect of the present invention is intended for an optical fiber of a single mode type including a core and a cladding.
As characteristics, a relative refraction index difference of the core and the cladding is increased to be greater than a relative refraction index difference of a communication-dedicated single mode optical fiber, and a core diameter is increased to be larger than a core diameter of the communication-dedicated single mode optical fiber. Thereby, a mode field diameter is set to be substantially the same as a mode field diameter of the communication-dedicated single mode optical fiber.
By thus increasing the relative refraction index difference of the core and the cladding to be greater than the relative refraction index difference of the communication-dedicated single mode optical fiber, confinement of light into the core is enhanced. Consequently, when the optical fiber is bent, it is suppressed that light in the core transmit to the cladding. As such, in comparison to the communication-dedicated optical fiber, the optical fiber reduces the bending loss. That is, the minimum radius of curvature of the optical fiber is less than the minimum radius of curvature of the communication-dedicated optical fiber.
However, when the relative refraction index difference of the optical fiber is increased to be greater than that of the communication-dedicated single mode optical fiber, the mode field diameter thereof becomes smaller than the mode field diameter of the communication-dedicated single mode optical fiber. As such, when the optical fiber is connected to the ordinary communication-dedicated single mode optical fiber, the connection loss is increased.
For this reason, according to the optical fiber of the first aspect of the present invention, the core diameter is increased to be larger than the core diameter of the communication-dedicated single mode optical fiber, whereby the mode field diameter is set to be substantially the same as the mode field diameter of the communication-dedicated single mode optical fiber. This arrangement prevents the connection loss from being increased when the optical fiber is connected to the communication-dedicated single mode optical fiber.
As described above, according to the optical fiber of the first aspect of the present invention, the bending loss is reduced, and the connection loss is not increased even when it is connected to the communication-dedicated single mode optical fiber. As such, the optical fiber is suitable to an environment including many portions where it is bent; and it can be optimally used with, for example, an optical-fiber wiring board.
It is noted that the first aspect of the present invention is realized by not considering the dispersion as an optical-fiber design parameter. For example, for the optical fiber to be used with a length of 10 m or shorter, the dispersion need not be taken into consideration as a design parameter.
Herein, the relative refraction index difference xcex94 [%] is preferably set to satisfy:
xcex940 less than xcex94xe2x89xa6xcex940+0.5[%],
where the relative refraction index difference of the communication-dedicated single mode optical fiber is represented by xcex940[%].
The setting is thus performed for the following reasons. Generally, when the relative refraction index difference xcex94 is increased, a cut-off wavelength is increased. However, the cut-off wavelength needs to be set to be less than a wavelength (for example, 1.3 xcexcm or 1.55 xcexcm) of propagation light that propagates through the optical fiber. Hence, when the relative refraction index difference xcex94 is set to be excessively large in comparison with the ordinary relative refraction index difference xcex940, control of the cut-off wavelength to a predetermined value becomes difficult. For this reason, the setting is preferably performed so that the deviation between the relative refraction index difference xcex94 of the optical fiber and the relative refraction index difference xcex940 of the communication-dedicated single mode optical fiber is within 0.5%.
The cut-off wavelength herein is a theoretical cut-off wavelength theoretically calculated according to the structure of the optical fiber. In comparison, a practical cut-off wavelength (effective cut-off wavelength) is less than the theoretical cut-off wavelength depending on, for example, the length of the optical fiber and the construction of other optical fibers connected to the front and the back of the optical fiber.
As such, the theoretical cut-off wavelength of the optical fiber may be set in such a manner that the effective cut-off wavelength is equal to or less than a wavelength of the propagation light.
For example, the theoretical cut-off wavelength xcexc [xcexcm] may be set to satisfy:
xcex less than xcexcxe2x89xa6xcex+0.05 [xcexcm],
where the wavelength of the propagation light is represented by xcex [xcexcm].
Thus, even with the optical fiber designed by setting the theoretical cut-off wavelength to be longer than the wavelength of the propagation light, the effective cut-off wavelength becomes less than the wavelength of the propagation light. On the other hand, even with the relative refraction index difference increased corresponding to the increase in the length of the theoretical cut-off wavelength, the mode field diameter of the optical fiber takes a desired value. Consequently, an optical fiber for which the bending loss is even more reduced can be obtained.
A second aspect of the present invention is intended for an optical-fiber wiring board including: an optical fiber of a single mode type including a core and a cladding; and a substrate on which the optical fiber is routed.
As characteristics of the above, the optical fiber is constructed in such a manner that a relative refraction index difference of the core and the cladding is increased to be greater than a relative refraction index difference of a communication-dedicated single mode optical fiber and a core diameter thereof is increased to be larger than a core diameter of the communication-dedicated single mode optical fiber. Thereby, a mode field diameter thereof is set to be substantially the same as a mode field diameter of the communication-dedicated single mode optical fiber.
The optical-fiber wiring board may include a curved portion where the optical fiber is routed to be in a circular arc shape.
In addition, the optical-fiber wiring board may include a crossover section where two optical fibers are routed to cross over one another on the substrate.
The substrate preferably has an adhesive layer for adhering the optical fiber.
The optical fibers on the optical-fiber wiring board may be routed on the substrate in such a manner as to perform matrix conversion of inputs of m ports (m represents a natural number) and n channels (n represents a natural number) into outputs of n ports and m channels.
As described above, since the optical fiber routed on the substrate of the optical-fiber wiring board has the relative refraction index difference that is greater than the relative refraction index difference of the communication-dedicated single mode optical fiber, the bending loss is reduced. Therefore, the bending loss is not increased even when the optical fiber is routed at a small radius of curvature on the substrate. The bending loss is not increased also in the crossover section where the two optical fibers cross over one another. Consequently, miniaturization can be implemented for the optical-fiber wiring board. Further, since the minimum radius of curvature formed when the optical fiber is routed is small, a fiber-routing pattern of the optical fiber on the optical-fiber wiring board can be changed to a more complex fiber-routing pattern.
Further, since the bending loss of the optical fiber is reduced, the performance of the optical-fiber wiring board is stabilized. Furthermore, for example, even in a case where the optical fiber routed on the substrate is bent following the substrate itself that is bent, the bending loss of the optical fiber is not increased. As such, the optical-fiber wiring board is capable of securely maintaining a predetermined performance.
Preferably, the optical fiber on the optical-fiber wiring board is constructed in such a manner that a relative refraction index difference xcex94 [%] thereof is set to satisfy:
xcex940 less than xcex94xe2x89xa6xcex940+0.5 [%],
where the relative refraction index difference of the communication-dedicated single mode optical fiber is represented by xcex940 [%].
A theoretical cut-off wavelength of the optical fiber is preferably set in such a manner that an effective cut-off wavelength is equal to or less than a wavelength of propagation light.
The theoretical cut-off wavelength xcexc [xcexcm] of the optical fiber is preferably set to satisfy:
xcex less than xcexcxe2x89xa6+0.05 [xcexcm],
where the wavelength of the propagation light is represented by xcex [xcexcm].
A third aspect of the present invention is intended for an optical-fiber wiring assembly constructed in such a manner that a communication-dedicated single mode optical fiber is connected to each of input and output ports of an optical-fiber wiring board including an optical fiber of a single mode type including: a core and a cladding; and a substrate on which the optical fiber is routed.
As characteristics of the above, the optical fiber is constructed in such a manner that a relative refraction index difference of the core and the cladding is increased to be greater than a relative refraction index difference of the communication-dedicated single mode optical fiber and a core diameter thereof is increased to be larger than a core diameter of the communication-dedicated single mode optical fiber, whereby a mode field diameter thereof is set to be substantially the same as a mode field diameter of the communication-dedicated single mode optical fiber.
As described above, the mode field diameter of the optical fiber and the mode field diameter of the communication-dedicated single mode optical fiber in the optical-fiber wiring assembly are substantially the same. Therefore, even when the communication-dedicated single mode optical fiber is connected to the optical fiber, the connection loss is not increased. As such, an ordinary communication-dedicated single mode optical fiber can be connected to each of input and output ports of the optical-fiber wiring board.